Process and system for manufacturing an encapsulated semiconductor device

ABSTRACT

A process for manufacturing a semiconductor device envisages the steps of: positioning a frame structure, provided with a supporting plate carrying a die of semiconductor material, within a molding cavity of a mold; and introducing encapsulating material within the molding cavity for the formation of a package, designed to encapsulate the die. The frame structure is further provided with a prolongation element mechanically coupled to the supporting plate inside the molding cavity and coming out of the molding cavity, and the process further envisages the steps of: controlling positioning of the supporting plate within the molding cavity with the aid of the prolongation element; and, during the step of introducing encapsulating material, separating and moving the prolongation element away from the supporting plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process and to a system formanufacturing an encapsulated semiconductor device; in particular, theensuing treatment will make reference, without this implying any loss ofgenerality, to the production by molding of a power package for asemiconductor device, of the full insulated type.

2. Discussion of the Related Art

Power semiconductor devices, for example power MOSFETs, are known thatcomprise a plastic package designed to encapsulate a die ofsemiconductor material integrating a corresponding integrated circuit,wherein the plastic package is commonly obtained by molding.

For example, FIG. 1 a and 1 b show a semiconductor device 1 (inparticular a power device) encapsulated in a package 2, made of plasticmaterial, for example epoxy resin, of the type known as JEDEC TO-220.The semiconductor device 1 comprises a die 3 of semiconductor materialand a leadframe 4 set at least partially within the package 2 anddesigned to support the die 3 within the same package 2, and to providethe electrical connection towards the outside of the integrated circuitwithin the die 3. The leadframe 4 comprises: a metal plate (known ingeneral as “die pad”) 5, set entirely within the package 2 and having atop surface 5 a, to which the die 3 is coupled (for example, viainterposition of adhesive material); and a plurality of leads 6, forexample three, which come out of the package 2. In a way notillustrated, the die pad 5 is made of a single piece with one of theleads 6 (in particular, with the lead set in a central position),consequently constituting an electrode of the semiconductor device 1,and the die 3 is connected electrically to the remaining leads 6 bymeans of bond wires, which extend from a respective contact pad, carriedby a top surface of the die 3 not in contact with the die pad 5, and arespective lead 6. The package 2 moreover has a through hole 7 at an endportion thereof (opposite to the one from which the leads 6 come out),for coupling, for example, by means of a screw or rivet, of thesemiconductor device 1 to a heat sink (not illustrated). In this regard,the die pad 5 transfers the heat generated in use by the circuitintegrated in the die 3 towards the aforesaid heat sink.

Given the need to ensure a good transfer of heat towards the heat sink,and (at least in the case of insulated packages) to insulate the die pad5 electrically from the outside of the package 2, during manufacture ofthe semiconductor device 1, and in particular of molding of the package2, a controlled thickness of the encapsulating material of the packageunderneath the die pad 5 needs to be guaranteed (in particular of thematerial in contact with a bottom surface 5 b of the die pad 5, oppositeto the top surface 5 a to which the die 3 is coupled). The thermal andelectrical performance of the resulting semiconductor device 1 can varyeven considerably according to the aforesaid thickness, which ismarkedly dependent on the technique for manufacturing the package 2, andin particular on the correct positioning of the leadframe 4 during themolding step. An incorrect alignment of the leadframe 4 with respect tothe mold used for the formation of the package 2 can cause a degradationof the thermal performance (in terms of thermal resistance R_(th)) ifthickness of the encapsulating material is greater than an upperspecification limit (USL), or the exposure of the bottom surface 5 b ofthe die pad 5 if thickness of the encapsulating material is considerablylower than a lower specification limit (LSL). If the aforesaid thicknessis smaller than the lower specification limit, an inadequate flow ofencapsulating material during molding may also occur, causing thecreation of voids at the backside of the package 2.

In a known manner, according to the molding technique and the resultingstructure of the package, power packages for semiconductor devices aredivided into “full molded” and “full insulated.” In both cases, the die3 is entirely coated with the encapsulating material, but in full moldedpackages areas of exposed metal may remain (for example, portions of thedie pad 5 may be accessible from the outside of the package 2), whereasin full insulated packages the total absence of exposed metal needs tobe guaranteed. It is evident that, especially in the case of fullinsulated packages, the presence of an excessively thin layer ofencapsulating material on the backside of the device can irreparablyjeopardize its performance.

It is consequently necessary to arrange the leadframe 4 and keep it in aproper and pre-set position within a corresponding mold during the stepof molding of the package 2, in particular during injection of theencapsulating material and its subsequent hardening (polymerization). Inthe past, a wide range of molding processes have been proposed, designedto address this need.

For example, one of the proposed techniques envisages the use of fixedpins (so-called “fixed-pin” technique), fixedly coupled to the mold, andsuch as to come to abut on opposed portions of the top and bottomsurfaces of the die pad 5 within the molding cavity, thus keeping thedie pad in a desired position upon closing of the mold. Once the moldingstep is terminated, as illustrated in FIG. 2, the package 2 has,however, voids 10, which leave the die pad 5 exposed, in positionscorresponding to the ones occupied by the fixed pins during molding.This technique can consequently be used for the production of fullmolded packages, but not for the production of full insulated packages.

In the case where the production of a full insulated package isrequired, the process described previously can be completed with a finaloff-line step (i.e., one distinct from and subsequent to the moldingstep) of filling of the voids 10 left by the fixed pins, with an epoxycompound (commonly known as “potting”), which is subsequently cured.FIG. 3 shows the resulting semiconductor device 1, in which reference 11designates the filling portions that totally close the voids 10. Theresulting process suffers, however, from a series of drawbacks, amongstwhich: the longer duration of the manufacturing method; the need foradditional equipment and the associated additional costs; and thepossibility of occurrence of reliability problems due to the fact thatthe epoxy compound is a material “external” to the encapsulatingmaterial that forms the body of the package.

To overcome these drawbacks, an alternative molding method has beenproposed, which envisages the use of retractable ejector pins. Indetail, in an initial step, FIG. 4 a, the leadframe 4 is inserted withina molding cavity 12 of a mold 13. An input channel (or gate) 14 is influid communication with the molding cavity 12 and enables theintroduction of encapsulating material. The leadframe 4 is kept in adesired position by means of the use of retractable pins 15, which arebrought into contact with the die pad 5 (as highlighted by the arrows inFIG. 4 a) so as to abut on a respective top surface 5 a or bottomsurface 5 b thereof. For this purpose, guides 16 are provided in themold 13, and the retractable pins 15 can slide within the guides 16 bythe action of suitable actuators (not illustrated). Subsequently (FIG. 4b), encapsulating material 17 is injected within the mold 13; theretractable pins 15 keep the die pad 5 in the proper position so as toguarantee the thickness required by specifications of the encapsulatingmaterial 17 on the backside of the package 2. Next (FIG. 4 c), once themolding cavity 12 is entirely filled with the encapsulating material,but before it polymerizes, the retractable pins 15 are retracted andmoved away from the die pad 5 (causing them to slide again in the guides16, as indicated by the arrows in FIG. 4 c). The pressure of injectionof the encapsulating material 17 is then increased so that it comes tooccupy the empty spaces left by the retractable pins 15, andsubsequently the required compactness of the package is achieved byhardening.

This method enables formation of a full insulated package with a goodaccuracy of the thickness of the encapsulating material 17 on the backof the leadframe 4. However, it has the problem of rapid wear of theretractable pins 15 and of the corresponding guide within the moldingcavity 12, due to the abrasive characteristics of the encapsulatingmaterial 17 used for the manufacturing of the package 2 (in general,epoxy resin containing an inorganic part, known as filler, with abrasiveproperties). This aspect has a negative impact both on the costs ofproduction and on the quality of the devices thus produced.

SUMMARY OF THE INVENTION

One aim of the present invention is consequently to provide a processthat enables the aforesaid disadvantages and problems to be overcome andin particular that enables a good precision in the control of thethickness of the encapsulating material to be obtained, as well ashaving reduced costs.

According to one aspect of the present invention, a process formanufacturing a semiconductor device is provided, comprising a processfor manufacturing a semiconductor device, comprising: positioning aframe structure, provided with a supporting plate carrying a die ofsemiconductor material, within a molding cavity of a mold; andintroducing encapsulating material within said molding cavity for theformation of a package, designed to encapsulate said die, wherein saidframe structure is provided with a prolongation element, mechanicallycoupled to said supporting plate inside said molding cavity and comingout of said molding cavity, and by further comprising: controllingpositioning of said supporting plate within said molding cavity with theaid of said prolongation element; and during said step of introducingencapsulating material, separating and moving said prolongation elementaway from said supporting plate.

According to another aspect of the present invention, a system formanufacturing a semiconductor device is provided, comprising a systemfor manufacturing a semiconductor device, comprising: a mold defining amolding cavity, designed to house a frame structure of saidsemiconductor device provided with a supporting plate carrying a die ofsemiconductor material; and an introduction unit for introduction ofencapsulating material within said molding cavity for the formation of apackage designed to encapsulate said die, wherein said frame structureis further provided with a prolongation element, mechanically coupled tosaid supporting plate inside said molding cavity and coming out of saidmolding cavity, and by further comprising: a positioning-control unit,configured to control positioning of said supporting plate within saidmolding cavity with the aid of said prolongation element; and anactuation unit, configured to separate and move said prolongationelement away from said supporting plate during introduction of saidencapsulating material.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, preferredembodiments thereof are now described, purely by way of non-limitingexample and with reference to the attached drawings, wherein:

FIG. 1 a is a schematic perspective view of an encapsulatedsemiconductor device of a known type;

FIG. 1 b is a cross-sectional view of the device of FIG. 1;

FIG. 2 is a cross-sectional view of a semiconductor device with fullmolded package of a known type;

FIG. 3 is a cross-sectional view of a semiconductor device with fullinsulated package of a known type;

FIGS. 4 a-4 c show sections of a further semiconductor device with fullinsulated package of a known type, in successive steps of acorresponding molding process;

FIGS. 5 a-5 d show sections of an encapsulated semiconductor device inaccordance with an embodiment of the present invention, in successivesteps of a corresponding process for manufacturing by molding; and

FIG. 6 is a simplified block diagram of a molding system according to afurther embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 5 a relates to an initial step of a molding process for asemiconductor device according to an embodiment of the presentinvention. In this and in the subsequent figures, parts that are similarto other described previously will be designated with the same referencenumbers, and will not be described again in detail. In particular, aleadframe 4 of the semiconductor device 1 is positioned within themolding cavity 12 of a mold 13 of a molding apparatus (of a known typethat is not herein described in detail). The leadframe 4 comprises a diepad 5, arranged totally within the molding cavity 12, and a plurality ofleads 6 (just one of which is illustrated in FIG. 5 a), which come outof the molding cavity 12. Conveniently, the die pad 5 and the leads 6have one and the same thickness, for example of a few millimeters, andare obtained by shaping and processing of one and the same ribbon ofmetal material, for example, copper.

The leadframe 4 further comprises a prolongation element 20 coupled tothe die pad 5 inside the molding cavity 12 and coming out on the outsideof the same molding cavity 12. In particular, the prolongation element20 comprises a connection portion 20 a, arranged within the moldingcavity 12, and a grip portion 20 b, arranged on the outside of themolding cavity 12. The connection portion 20 a is mechanically connectedto the die pad 5 at an area with facilitated breaking (facilitatedbreaking area 22). In a preferred embodiment, the prolongation element20 is made of a single piece with the die pad 5 (in particular, startingfrom the same ribbon of metal material), and the facilitated breakingarea 22 is an area of weakening of the same metal ribbon situatedbetween the die pad 5 and the connection portion 20 a. For instance, thefacilitated breaking area 22 is obtained by means of removal of materialand consequent reduction of the thickness of the aforesaid metal ribbon.

The grip portion 20 b, which comes out of the molding cavity 12, enablespositioning, in a desired way, the die pad 5 of the leadframe 4 duringmolding operations. In detail, both a first end of the die pad 5,connected integrally to the lead 6 coming out of the molding cavity 12,and a second diametrally opposite end of the same die pad 5, connectedmechanically to the grip portion 20 b, also coming out of the moldingcavity 12, have a position established in a precise way by clamping,between a top half 13 a and a bottom half 13 b of the mold 13, of theleads 6 and grip portion 20 b, respectively. The die pad 5 is hencecorrectly and stably positioned, in particular with an end portionthereof centered with respect to the molding cavity 12 of the mold 13,ensuring that a repeatable and accurate thickness of encapsulatingmaterial is obtained on the backside of the die pad 5 (this thickness isdesignated by h in FIG. 5 a, and is indicated by the arrows).

In a subsequent step of the molding process (FIG. 5 b), theencapsulating material 17, in this case an epoxy resin (or otherelectrically non-conductive thermosetting plastic material), is injectedunder pressure within the molding cavity 12, through an input channel14, set for example at the point where the leads 6 come out of themolding cavity 12. In this step, the molding cavity 12 is filledentirely with the encapsulating material, which has not, however, yetreached the desired compactness (the thermosetting process is still inprogress).

Next (FIG. 5 c), the grip portion 20 b is pulled mechanically, moving itaway from the molding cavity 12 (in the direction indicated by thearrow), for example by means of an appropriate hydraulic actuator (notillustrated). This operation causes breaking of the facilitated breakingarea 22, the separation of the connection portion 20 a from the die pad5, and the formation of an empty space 23, without encapsulatingmaterial, within the molding cavity 12. In this step, the prolongationelement 20 is not removed altogether from the mold 13, but displaceduntil it is positioned exactly at the end of the molding cavity 12,closing it laterally (in other words, the displacement stopssubstantially when the connection portion 20 a comes out entirely of themolding cavity 12). At this point, the transfer of the encapsulatingmaterial 17 continues so as to fill the empty space 23 previouslyoccupied by the connection portion 20 a, and then proceeds until thedesired compactness is reached (completion of the polymerizationprocess). During this step, the presence of the prolongation element 20at the end of the molding cavity 12 prevents undesirable exit of theencapsulating material 17 from the molding cavity 12.

Next (FIG. 5 d), once the process for molding of the package 2 of thesemiconductor device 1 is terminated, the prolongation element 20 isfully removed from the molding cavity 12 and from the molding apparatus.The mold 13 is opened, separating the top half 13 a from the bottom half13 b so as to enable extraction of the semiconductor device 1, which isnow encapsulated and ready for subsequent processing steps.

As illustrated in FIG. 6, a molding system 30 for implementation of theprocess previously described comprises: the mold 13, designed to receivethe leadframe 4 and the corresponding die 3 for their encapsulation withthe encapsulating material 17; an actuation unit 32, designed toco-operate with the leadframe 4 during the molding operations, and inparticular to separate and move the prolongation element 20 away fromthe die pad 5; an introduction unit 34, designed to control introductionunder pressure of the encapsulating material 17 within the moldingcavity 12 of the mold 13; and a position-control unit 36, designed toco-operate with the prolongation element 20 for control of positioningof the die pad 5 within the molding cavity 12, during the initial stepsof the molding process.

The process described has a number of advantages.

In particular, it enables a semiconductor device 1 to be obtained, whichcomprises a package 2 of a full insulated type, made up of just oneplastic encapsulation material, in a simple and economicallyadvantageous way (given that no additional equipment is required),without the drawbacks of the known art (and in particular without thedisadvantages associated to the use of retractable ejector pins or thelike, and with a better precision with respect to the use of fixedpins).

The process described ensures an accurate and controlled positioning ofthe leadframe 4 during molding, and consequently a repeatable andprecise thickness of the encapsulating material 17 can be obtained onthe backside of the package 2. Semiconductor devices 1 are thus obtainedwith pre-established and repeatable mechanical and electricalcharacteristics, preventing an increase in the defectiveness and adecrease in the performance.

Finally, it is clear that modifications and variations can be made towhat is described and illustrated herein, without thereby departing fromthe scope of the present invention, as defined in the annexed claims.

In particular, the described process can be advantageously used withfurther types of packages, in which control of the thickness of theencapsulating material is required. For instance, it can be used withnon-insulated packages, or with any type of package (also for signalapplications, and not power applications) requiring a completeencapsulation of the leadframe in the compound of encapsulatingmaterial.

It is clear that different shapes can be envisaged for the package 2, aslikewise there may be envisaged a different number of leads 6 (the sameelectrical connections between the die 3 and the leads 6 may vary).

Furthermore, the connection portion 20 a of the prolongation element 20may be mechanically coupled to the die pad 5 in a different way througha corresponding area with facilitated breaking, without being made of asingle piece with the die pad; the molding method previously describedremains in any case substantially unchanged.

1. A process for manufacturing a semiconductor device, comprising:positioning a frame structure, provided with a supporting plate carryinga die of semiconductor material, within a molding cavity of a mold; andintroducing encapsulating material within said molding cavity for theformation of a package, designed to encapsulate said die, wherein saidframe structure is provided with a prolongation element, mechanicallycoupled to said supporting plate inside said molding cavity and comingout of said molding cavity, and by further comprising: controllingpositioning of said supporting plate within said molding cavity with theaid of said prolongation element; and during said step of introducingencapsulating material, separating and moving said prolongation elementaway from said supporting plate.
 2. The process according to claim 1,wherein said prolongation element is coupled to said supporting plate ata facilitated breaking area, said separating comprising breaking saidfacilitated breaking area.
 3. The process according to claim 2, whereinsaid prolongation element is made integrally with said supporting plate,and said facilitated breaking area corresponds to an area of weakening,set between said supporting plate and said prolongation element.
 4. Theprocess according to claim 1, wherein said moving said prolongationelement away from said supporting plate comprises forming an empty spacewithout said encapsulating material within said molding cavity, placedin contact with said supporting plate; and said introducingencapsulating material further comprises filling said empty space. 5.The process according to claim 4, wherein said encapsulating materialcomprises thermosetting material, in particular an epoxy resin; saidstep of introducing encapsulating material further comprising hardeningsaid encapsulating material after said filling said empty space.
 6. Theprocess according to claim 1, wherein said separating said prolongationelement from said supporting plate comprises pulling said prolongationelement from a grip portion thereof set on the outside of said moldingcavity.
 7. The process according to claim 1, wherein said moving saidprolongation element away from said supporting plate comprisesextracting said prolongation element from said molding cavity.
 8. Theprocess according to claim 7, wherein said extracting comprises bringingsaid prolongation element flush with an end of said molding cavity so asto close said molding cavity laterally; further comprising, afterintroducing encapsulating material, removing said prolongation elementcompletely from said mold.
 9. The process according to claim 1, whereinsaid controlling positioning comprises clamping said prolongationelement between a first half and a second half of said mold.
 10. Theprocess according to claim 9, wherein said clamping comprises centeringa portion of said supporting plate with respect to said molding cavity.11. The process according to claim 1, wherein said die is coupled to atop surface of said supporting plate; and said controlling positioningcomprises controlling a thickness of said encapsulating material on abottom surface of said supporting plate not in contact with said die.12. The process according to claim 1, wherein said semiconductor deviceis a power device, and said package is a power package, configured tocoat and fully insulate said supporting plate and said die.
 13. A systemfor manufacturing a semiconductor device, comprising: a mold defining amolding cavity, designed to house a frame structure of saidsemiconductor device provided with a supporting plate carrying a die ofsemiconductor material; and an introduction unit for introduction ofencapsulating material within said molding cavity for the formation of apackage designed to encapsulate said die, wherein said frame structureis further provided with a prolongation element, mechanically coupled tosaid supporting plate inside said molding cavity and coming out of saidmolding cavity, and by further comprising: a positioning-control unit,configured to control positioning of said supporting plate within saidmolding cavity with the aid of said prolongation element; and anactuation unit, configured to separate and move said prolongationelement away from said supporting plate during introduction of saidencapsulating material.
 14. The system according to claim 13, whereinsaid prolongation element is coupled to said supporting plate at afacilitated breaking area, said actuation unit being configured to breaksaid facilitated breaking area.
 15. The system according to claim 14,wherein said prolongation element is made integrally with saidsupporting plate, and said facilitated breaking area corresponds to anarea of weakening, set between said supporting plate and saidprolongation element.
 16. The system according to claim 13, wherein saidactuation unit is further configured to form an empty space without saidencapsulating material within said molding cavity, placed in contactwith said supporting plate; and said introduction unit is furtherconfigured to fill said empty space with said encapsulating material.17. The system according to claim 16, wherein said encapsulatingmaterial comprises thermosetting material, in particular an epoxy resin,and said introduction unit is configured to cause hardening of saidencapsulating material after filling of said empty space.
 18. The systemaccording to claim 13, wherein said actuation unit is further configuredto pull said prolongation element by a grip portion thereof, set on theoutside of said molding cavity.
 19. The system according to claim 13,wherein said actuation unit is further configured to extract saidprolongation element from said molding cavity.
 20. The system accordingto claim 19, wherein said actuation unit is further configured to bringsaid prolongation element flush with one end of said molding cavity soas to close said molding cavity laterally, and, after introduction ofsaid encapsulating material, to remove said prolongation elementcompletely from said mold.
 21. The system according to claim 13, whereinsaid mold comprises a first half and a second half, and saidpositioning-control unit is configured to cause clamping of saidprolongation element between said first half and said second half ofsaid mold, in particular so as to center a portion of said supportingplate with respect to said molding cavity.
 22. The system according toclaim 13, wherein said die is coupled to a top surface of saidsupporting plate, and said positioning-control unit is furtherconfigured to control a thickness of said insulating material on abottom surface of said supporting plate not in contact with said die.23. The system according to claim 13, wherein said semiconductor deviceis a power device, and said package is a power package, configured tocoat and fully insulate said supporting plate and said die.